1. Field of the Invention
This invention relates to an apparatus for atomic layer deposition of material(s) on a substrate.
2. Description of the Art
Atomic layer deposition (“ALD”) is a thin film deposition technique that offers extremely precise control over the thickness of a layer of a compound material deposited on a substrate. As the name implies, the film growth in ALD is layer by layer, which allows the deposition of extremely thin, conformal coatings that are also free of grain boundaries and pinholes. Deposition of this coating is typically done through the application of two molecular precursors. The surface of the substrate is exposed to a first precursor (“precursor I”) molecule, which reacts chemically with the surface. This reaction is self-limiting and proceeds until there is a uniform monolayer coating of reacted precursor I covering the surface. The surface is then exposed to a second precursor (“precursor II”), which reacts chemically with the surface coated with precursor I to form the desired compound. As before, the reaction is self limiting, and the result is a completed monolayer coating of reacted precursor II covering the surface, and therefore a completed monolayer of the desired compound material.
The process can then be repeated, exposing the surface first to precursor I and then to precursor II, until a coating of the desired thickness has been formed. Since each completed I-II layer has a thickness on the order of 0.1 nm, very thin layers, with a very precisely controlled thickness are possible.
Historically, ALD has been carried out by placing the substrate to be coated in a vacuum chamber and introducing a low pressure carrier gas containing some small percentage of precursor, also in the gas phase. However, because the time to completely purge the precursors from the deposition chamber can be long, ALD has typically been regarded as a slow process.
An alternative form of ALD coating head is known that allows deposition at much higher rates. In this head arrangement the precursor gases (again, precursor molecules in an inert carrier gas) are delivered by long narrow channels, and these channels alternate with vacuum uptake channels and purge gas channels. The head is then traversed across the substrate to be coated in a direction perpendicular to the long axis of the output channels (or alternatively held in one position while the substrate is translated underneath it). U.S. Published Patent Application 2008/166,880 (Levy) is representative of the structure of such a head.
The head disclosed in this referenced published application requires that the separation between the head and the substrate be very small (˜thirty microns) and very closely controlled. In fact, jets of gas emanating from the face of the device are used as a means to float the coating head, in a manner analogous to a hovercraft, over the substrate to be coated.
In view of the foregoing it is believed to be advantageous to provide an apparatus for ALD coating of a substrate that is not sensitive to the precise distance between the coating head and the substrate, but is, instead, independent of the separation between the head and the substrate and tolerant of dimensional variations in that separation. In that way, no extraordinary measures would be needed to keep this separation distance fixed. In particular, it is believed to be advantageous not to require the gases exiting the head to do double duty: i.e., the gases should not be required to serve the function of maintaining a fixed separation at the cost of compromising the main function of the device, the deposition of an ALD coating.